How to Size a Pump for Sewer, Stormwater and Infrastructure Bypass Projects

When contractors need to maintain flow during sewer rehabilitation, stormwater works, pipeline repairs, or infrastructure upgrades, selecting the correct temporary pump is critical.

An undersized pump may fail to keep up with incoming flow, while an oversized pump can increase fuel consumption, operating costs, and system complexity.

For contractors working across the UAE, understanding the basic principles of pump sizing can help create a more reliable and efficient temporary bypass system.

Arabian Equipment Rental (AER) provides temporary pumping and equipment rental solutions for construction, infrastructure, drainage, sewer, and emergency applications across the UAE.

What Does Pump Sizing Mean?

Pump sizing is the process of selecting a pump that can deliver the required flow rate at the required total dynamic head (TDH) while handling the characteristics of the fluid and the conditions at the site.

For a bypass project, the basic questions are:

How much water or wastewater needs to be moved?

How much resistance will the pumping system create?

Two key values are therefore required:

• Flow rate — Q

• Total Dynamic Head — TDH

The selected pump must be capable of delivering the required flow at the calculated head.

1. Determine the Required Flow Rate

The first step is establishing how much fluid the bypass system needs to handle.

Flow may be expressed in:

• Litres per second (L/s)

• Cubic metres per hour (m³/h)

• Litres per minute (L/min)

• Gallons per minute (GPM)

For example, if a sewer system has an expected peak flow of 100 L/s, the bypass pumping system needs to accommodate that peak flow rather than simply the average daily flow.

Consider Peak Flow

Using average flow alone can result in inadequate pumping capacity.

Contractors should consider:

• Normal operating flow

• Peak flow

• Wet-weather flow

• Stormwater inflow

• Upstream connections

• Seasonal variations

• Potential infiltration

• Changes expected during construction

For stormwater projects, rainfall intensity and catchment characteristics can have a significant effect on the required pumping capacity.

2. Calculate Total Dynamic Head

Knowing the required flow is only half of the pump-sizing calculation.

The pump must also overcome the resistance created by the pumping system. This is known as Total Dynamic Head (TDH).

In a simplified system, TDH can be considered in terms of:

Static Head + Friction Losses + Pressure or Discharge Requirements

Static Head

Static head is the elevation difference between the pump suction level and the discharge point.

For example, if water must be pumped from an excavation at a lower elevation to a discharge point 8 metres higher, the system has approximately 8 metres of static elevation head before friction losses are considered.

Friction Loss

Fluid flowing through pipes and hoses experiences resistance.

Friction losses depend on factors such as:

• Pipe diameter

• Pipe length

• Flow velocity

• Pipe material

• Number of bends

• Valves

• Tees

• Reducers

• Other fittings

A long, small-diameter discharge line can create substantially more friction loss than a short, larger-diameter line.

This is why pump selection should consider the complete temporary pumping system, not just the pump itself.

3. Understand the Pump Curve

Every pump has an operating range. A pump curve shows the relationship between flow rate and head.

As the required head increases, the achievable flow generally decreases.

For example, a pump advertised as capable of 500 m³/h may achieve that flow only at a relatively low head. At a higher head, its actual flow may be significantly lower.

Contractors should therefore avoid selecting equipment based only on a pump's maximum advertised flow.

Instead, the required operating point should be identified based on:

Required Flow + Required Head

The pump curve should then be checked to confirm that the selected pump can provide the required flow at the calculated head and operate within the manufacturer's recommended range.

4. Select the Correct Pipe Diameter

Pump sizing and pipe sizing should be considered together.

A discharge pipe that is too small can create excessive friction loss and reduce pump performance.

Larger pipework can reduce friction losses but may increase:

• Material requirements

• Transportation requirements

• Installation time

• Space requirements

For temporary bypass projects, the appropriate pipe diameter depends on the required flow, pumping distance, head, site layout, and available equipment.

AER can help contractors assess temporary pumping equipment and pipework requirements based on project conditions.

5. Consider Solids Handling for Sewer Bypass Projects

Sewer bypass pumping is different from clean-water pumping because wastewater may contain:

• Solids

• Rags

• Fibrous materials

• Debris

• Sediment

• Other suspended material

Using equipment that is not suitable for wastewater can increase the risk of clogging, reduced performance, or downtime.

When selecting a sewer bypass pump, contractors should consider:

• Solids passage

• Impeller design

• Pump type

• Suction conditions

• Expected debris

• Cleaning and maintenance requirements

The pump should be selected according to the actual characteristics of the wastewater rather than simply its required flow rate.

6. Consider Stormwater Conditions

Stormwater bypass projects can present a different challenge because flows may change rapidly.

A system designed for normal conditions may become inadequate during a significant rainfall event.

For stormwater applications, contractors may need to consider:

• Catchment area

• Expected rainfall

• Rainfall intensity

• Inflow rate

• Existing drainage capacity

• Pumping duration

• Discharge location

• Available storage

Depending on the project, temporary storage combined with pumping capacity may form part of the overall bypass strategy.

7. Check Suction Conditions

Pump performance is also affected by conditions on the suction side.

Important factors include:

• Suction lift

• Suction hose diameter

• Suction hose length

• Water level

• Pump location

• Air leaks

• Strainers and fittings

Where applicable, contractors should consider Net Positive Suction Head (NPSH) requirements to help avoid cavitation and unstable pump operation.

Positioning the pump as close as practical to the source can sometimes simplify the suction arrangement and improve operating conditions.

8. Plan for Standby Capacity

For critical sewer and infrastructure bypass projects, the consequences of pump failure can be significant.

Contractors should therefore assess whether standby pumping capacity is required.

Depending on the project, a bypass system may include:

• Duty pump

• Standby pump

• Additional emergency pump

• Backup generator

• Automatic controls

• High-level alarms

• Remote monitoring

The required level of redundancy depends on the project's risk, flow characteristics, duration, and consequences of failure.

9. Consider UAE Site Conditions

Temporary pumping projects in the UAE may involve challenging site conditions that should be considered during equipment selection and planning.

High Ambient Temperatures

High temperatures can affect engines, fuel systems, electrical equipment, hoses, and working conditions around the equipment.

Dust and Sand

Construction environments can expose pumps and engines to dust and sand, making inspection and maintenance important.

Restricted Urban Sites

Projects in Dubai, Abu Dhabi, Sharjah, and other developed areas may have limited space for pumps, pipework, generators, and access vehicles.

Traffic and Pedestrian Movement

Temporary discharge lines may need to cross roads, access routes, or pedestrian areas. These requirements should be incorporated into site planning and safety arrangements.

10. Example Pump-Sizing Calculation

Consider a simplified bypass requirement:

Required flow: 200 m³/h

Static elevation: 6 m

Estimated pipe and fitting losses: 9 m

Required discharge pressure/head allowance: 2 m

The estimated total dynamic head would be:

TDH = 6 + 9 + 2 = 17 m

The contractor would therefore look for a pump capable of delivering approximately:

200 m³/h at 17 m TDH

The pump curve should then be checked to confirm the actual operating point and whether the selected pump can meet the required duty.

This example is simplified. Actual projects should use measured site information and appropriate hydraulic calculations before final equipment selection.

Common Pump-Sizing Mistakes

Choosing a Pump Based Only on Maximum Flow

A pump's maximum flow rating does not necessarily represent the flow it can achieve at the required head.

Ignoring Pipe Friction

Long discharge routes, smaller pipe diameters, bends, valves, and other fittings can create significant pressure losses.

Using Average Instead of Peak Flow

Peak inflows can exceed normal operating conditions, particularly during stormwater events or changing site conditions.

Forgetting Solids Handling

Sewer applications require equipment capable of handling the expected wastewater characteristics and solids.

Not Providing Standby Capacity

A pump failure can quickly become a major site problem if adequate backup capacity has not been considered.

Failing to Consider the Complete System

The pump, suction hose, discharge pipe, fittings, valves, power supply, controls, and discharge point should be considered as one integrated pumping system.

Pump-Sizing Checklist for Contractors

Before requesting temporary pumping equipment, contractors should gather the following information:

• Required flow rate

• Peak flow rate

• Static elevation difference

• Suction lift

• Discharge distance

• Pipe or hose diameter

• Number of bends and fittings

• Fluid type

• Expected solids or debris

• Required pumping duration

• Available electrical power

• Fuel requirements

• Site access restrictions

• Required standby capacity

• Discharge location

• Emergency pumping requirements

The more accurate the project information, the easier it is to assess the pumping requirements and identify suitable equipment.

Choosing the Right Pump for Your Bypass Project

Pmp sizing is not simply about choosing the biggest pump available.

The selected equipment needs to match the project's flow, head, fluid characteristics, pipework, site conditions, operating period, and required reliability.

For contractors, early planning can help reduce the risk of insufficient capacity, unexpected downtime, and unnecessary operating costs.

Arabian Equipment Rental (AER) supports contractor's with temporary pumping and equipment rental solutions for sewer bypass, stormwater management, dewatering, infrastructure construction, maintenance, and emergency applications across the UAE.

Need Help Selecting a Temporary Pump?

If you are planning a sewer, stormwater, pipeline, or infrastructure bypass project, provide AER with key project information such as:

• Required flow

• Pumping distance

• Elevation difference

• Fluid type

• Expected solids or debris

• Site conditions

• Required pumping duration

This information can help the AER team assess the pumping requirements and identify a suitable temporary pumping configuration for the project.